21611561 has 4 divisors (see below), whose sum is σ = 21723732. Its totient is φ = 21499392.

The previous prime is 21611549. The next prime is 21611567. The reversal of 21611561 is 16511612.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 1755625 + 19855936 = 1325^2 + 4456^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21611561 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21611567) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 55796 + ... + 56181.

It is an arithmetic number, because the mean of its divisors is an integer number (5430933).

Almost surely, 2^21611561 is an apocalyptic number.

It is an amenable number.

21611561 is a deficient number, since it is larger than the sum of its proper divisors (112171).

21611561 is a wasteful number, since it uses less digits than its factorization.

21611561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 112170.

The product of its digits is 360, while the sum is 23.

The square root of 21611561 is about 4648.8236146363. The cubic root of 21611561 is about 278.5450076442.

The spelling of 21611561 in words is "twenty-one million, six hundred eleven thousand, five hundred sixty-one".